Electrochemical gradients across Locusta Malpighian tubules

The use of conventional microelectrodes has revealed that the basal membrane potential (Em_b) and the transepithelial potential (Tm) in Locusta Malpighian tubules are -39.4 mV and +4.4 mV respectively. No change in Em_b was noted after stimulation by diuretic hormone (DH) or cAMP (5 mM), suggesting electroneutral coupling of ions entering the cell. Intracellular concentrations of potassium and chloride measured by ion selective microelectrodes (ISMs) are 95 mM and 51 mM respectively, and intraluminal potassium measured by direct impalement using ISMs is 139 mM. Luminal sodium and chloride concentrations, measured indirectly, by ISMs in the secreted fluid, are 20 mM and 203 mM respectively in unstimulated tubules. Using the data for the electrical and chemical gradients it has been possible to speculate how the different ions enter and exit Locusta tubule cells. A hypothetical model has been formulated in which it is suggested that K⁺ and Na⁺ can enter across the basal membrane passively but require an energy dependent mechanism to exit across the apical membrane. Conversely Cl^- must enter by an energy dependent mechanism but can exit down a favourable electrical gradient.